A number of different schemes are known for encoding, transmitting and decoding identification signals from RF-ID tags. However, these schemes are generally incompatible, therefore requiring proprietary readers to accept encoded transmissions from tags of the same vendor. Even where the transmission scheme is not proprietary, there is no standardization in the various RF-ID applications.
These RF-ID tags comprise, at a minimum, an antenna and a signal transforming device, some known devices are very complex. There are two particular types of passive RF-ID tags which are used. A first type includes an electronic circuit, e.g., CMOS, to store digital ID data which is then modulated onto a received signal by means of an RF circuit, e.g., a GaAs MESFET, transistor or controlled diode. Power for the data storage and modulating circuit may be derived from an interrogating RF beam or another power source, and power for the transmission itself is also derived from the beam. In this type of system, the interrogating RF beam is generally of fixed frequency, with the resulting modulated signal at the same or a different frequency, with AM, FM, PSK, QAM or another known modulation scheme employed. In order to provide separation between the received and transmitted signals, the modulated output may be, for example, a harmonic of the interrogating RF beam. Such a system is disclosed in U.S. Pat. No. 4,739,328, expressly incorporated herein by reference.
A known RF-ID interrogation system provides an interrogation signal which incorporates phase diversity, i.e., a phase which periodically switches between 0.degree. and 90.degree. so that a null condition is not maintained for a period which would prevent RF-ID tag readout with a homodyne receiver. See, U.S. Pat. No. 3,984,835, incorporated herein by reference.
Likewise, a known system, described in U.S. Pat. No. 4,888,591, incorporated herein by reference, provides a semiconductor memory tag which is interrogated with a direct sequence spread spectrum signal, which allows discrimination of received signals based on signal return delay. By employing a direct sequence spread spectrum having a decreasing correlation of a return signal with the interrogation signal as delay increases, more distant signals may be selectively filtered. This system employs a homodyne detection technique with a dual balanced mixer.
A second type of RF-ID tag includes a surface acoustic wave device, in which an identification code is provided as a characteristic time-domain reflection pattern in a retransmitted signal, in a system which generally requires that the signal emitted from an exciting antenna be non-stationary with respect to a signal received from the tag. This ensures that the reflected signal pattern is distinguished from the emitted signal. In such a device, received RF energy, possibly with harmonic conversion, is emitted onto a piezoelectric substrate as an acoustic wave with a first interdigital electrode system, from whence it travels through the substrate, interacting with reflector elements in the path of the wave, and a portion of the acoustic wave is ultimately received by the interdigital electrode system and retransmitted. These devices do not require a semiconductor memory. The propagation velocity of an acoustic wave in a surface acoustic wave device is slow as compared to the free space propagation velocity of a radio wave. Thus, assuming that the time for transmission between the radio frequency interrogation system is short as compared to the acoustic delay, the interrogation frequency should change such that a return signal having a minimum delay may be distinguished, and the interrogation frequency should not return to that frequency for a period longer than the maximum acoustic delay period. Generally, such systems are interrogated with a pulse transmitter or chirp frequency system.
Systems for interrogating a passive transponder employing acoustic wave devices, carrying amplitude and/or phase-encoded information are disclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein by reference. The tags interact with an interrogator/receiver apparatus which transmits a first signal to, and receives a second signal from the remote transponder, generally as a radio wave signal. The transponder thus modifies the interrogation signal and emits encoded information which is received by the interrogator/receiver apparatus.
Because the encoded information normally includes an identification code which is unique or quasi-unique to each transponder, and because the transponders of this type are relatively light weight and small and may be easily attached to other objects to be identified, the transponders are sometimes referred to as "labels" or "tags". The entire system, including the interrogator/receiver apparatus and one or more transponders, which may be active or passive, is therefore often referred to as a "passive interrogator label system" or "PILS".
Other passive interrogator label systems are disclosed in the U.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217.
In its simplest form, the systems disclosed in these patents include a radio frequency transmitter capable of transmitting RF pulses of electromagnetic energy. These pulses are received at the antenna of a passive transponder and applied to a piezoelectric "launch" transducer adapted to convert the electrical energy received from the antenna into acoustic wave energy in the piezoelectric material. Upon receipt of a pulse, an acoustic wave is generated within the piezoelectric material and transmitted along a defined acoustic path. This acoustic wave may be modified along its path, such as by reflection, attenuation, variable delay, and interaction with other transducers.
When an acoustic wave pulse is reconverted into an electrical signal it is supplied to an antenna on the transponder and transmitted as RF electromagnetic energy. This energy is received at a receiver and decoder, preferably at the same location as the interrogating transmitter, and the information contained in this response to an interrogation is decoded. The tag typically has but a single antenna, used for both receiving the interrogation pulse and emitting an information bearing signal.
In general, a passive interrogator label system includes an "interrogator" for transmitting a first radio frequency signal; at least one transponder which receives this first signal, processes it and sends back a second radio frequency signal containing encoded information; and a receiver, normally positioned proximate to or integrated with the interrogator, for receiving the second signal and decoding the transponder-encoded information.
Known technologies allow separate interrogation systems to operate in close proximity, for example by employing directional antennas and employing encoded transmissions, such as a direct sequence spread spectrum signal, which has reduced self-correlation as relative delay increases, thus differentiating more distant signals.
In known passive transponder systems, the encoded information is retrieved by a single interrogation cycle, representing the state of the tag, or obtained as an inherent temporal signature of an emitted signal due to internal time delays.
In the acoustic wave tags described above, the interrogator transmits a first signal having a first frequency that successively assumes a plurality of frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905-925 MHz, referred to herein as the nominal 915 MHz band, a frequency band that may be available. The response of the tag to excitation at any given frequency is distinguishable from the response at other frequencies. Further, because the frequency changes over time, the received response of the tag, delayed due to the internal structures, may be at a different frequency than the simultaneously emitted signal, thus reducing interference.
Passive transponder encoding schemes include control over interrogation signal transfer function H(s), including the delay functions f(z). These functions therefore typically generate a return signal in the same band as the interrogation signal. Since the return signal is mixed with the interrogation signal, the difference between the two will generally define the information signal, along with possible interference and noise. By controlling the rate of change of the interrogation signal frequency with respect to a maximum round trip propagation delay, including internal delay, as well as possible Doppler shift, the maximum bandwidth of the demodulated signal may be controlled.
The following references are hereby expressly incorporated by reference for their disclosure of RF modulation techniques, transponder systems, information encoding schemes, transponder antenna and transceiver systems, excitation/interrogation systems, and applications of such systems: U.S. Pat. Nos. 2,193,102; 2,602,160; 2,774,060; 2,943,189; 2,986,631; 3,025,516; 3,090,042; 3,206,746; 3,270,338; 3,283,260; 3,379,992; 3,412,334; 3,480,951; 3,480,952; 3,500,399; 3,518,415; 3,566,315; 3,602,881; 3,631,484; 3,632,876; 3,699,479; 3,713,148; 3,718,899; 3,728,632; 3,754,250; 3,798,641; 3,798,642; 3,801,911; 3,839,717; 3,859,624; 3,878,528; 3,887,925; 3,914,762; 3,927,389; 3,938,146; 3,944,928; 3,964,024; 3,980,960; 3,984,835; 4,001,834; 4,019,181; 4,038,653; 4,042,906; 4,067,016; 4,068,211; 4,068,232; 4,069,472; 4,075,632; 4,086,504; 4,114,151; 4,123,754; 4,135,191; 4,169,264; 4,197,502; 4,207,518; 4,209,785; 4,218,680; 4,242,661; 4,287,596; 4,298,878; 4,303,904; 4,313,118; 4,322,686; 4,328,495; 4,333,078; 4,338,587; 4,345,253; 4,358,765; 4,360,810; 4,364,043; 4,370,653; 4,370,653; 4,388,524; 4,390,880; 4,471,216; 4,472,717; 4,473,851; 4,498,085; 4,546,241; 4,549,075; 4,550,444; 4,551,725; 4,555,618; 4,573,056; 4,599,736; 4,604,622; 4,605,012; 4,617,677; 4,627,075; 4,641,374; 4,647,849; 4,654,512; 4,658,263; 4,739,328; 4,740,792; 4,759,063; 4,782,345; 4,786,907; 4,791,283; 4,795,898; 4,798,322; 4,799,059; 4,816,839; 4,835,377; 4,849,615; 4,853,705; 4,864,158; 4,870,419; 4,870,604; 4,877,501; 4,888,591; 4,912,471; 4,926,480; 4,937,581; 4,951,049; 4,955,038; 4,999,636; 5,030,807; 5,055,659; 5,086,389; 5,109,152; 5,131,039; 5,144,553; 5,163,098; 5,193,114; 5,193,210; 5,310,999; 5,479,160; and 5,485,520. In addition, foreign patents CH346388; DE1295424; DE2926836; DE969289; EP0207020; FR2260115; GB1130050; GB1168509; GB1187130; GB2103408; GB2247096; GB774797; GB987868; JP0138447; JP0189467; JP116054; JP5927278; and NE1566716, as well as the following references: "IBM Technical Disclosure Bulletin", (vol. 20, No. 7; 12/77), pp. 2525-2526.; "IEEE Transactions on Vehicular Technology", (vol. VT-26, No. 1), 2/77; p. 35.; A. R. Koelle et al. "Short-Range Radio-Telemetry for Electronic Identification using Modulated RF Backscatter", by A. (Proc. of IEEE, 8/75; pp. 1260-1261).; Baldwin et al., "Electronic Animal . . . Monitoring", 1973.; Electronic Letters, December 1975, vol. 11, pp. 642-643.; Encyclopedia. of Science and Technology; vol. 8, pp. 644-647 (1982).; Federal Information Processing Standards Publication 4A, Jan. 15, 1977, Specifications for the Data Encryption Standard.; IEEE Transactions, Henoch et al., vol. MTTT-19, No. 1, January 1971.; IEEE Transactions, Jaffe et al., pp. 371-378, May 1965.; RE Transactions, Harrington, pp. 165-174, May 1962.; IRE Transactions, Rutz, pp. 158-161, March 1961.; J. Lenk, Handbook of Microprocessors, Microcomputers and Minicomputers; p. 51 (1979).; Koelle et al., "Electronic Identification . . . Monitoring", 7/73 to 6/74, pp. 1-5.; P. Lorrain et al., E M Fields and Waves; Appendix A, (1970).; Proceedings of IRE, March 1961, pp. 634-635.; R. Graf, Dictionary of Electronics; p. 386, (1974).; RCA Review, vol. 34, 12/73, Klensch et al., pp. 566-579.; RCA Review, Sterzer, 6/74, vol. 35, pp. 167-175.; Reports on Research, September--October 1977, vol. 5, No. 2. each of which is expressly incorporated herein by reference.